One of the more common problems in the operation of a radar system is the reduction of interference in the radar receiver. Interfering signals may be due to reflection of the transmitted radar signal from directions not of present interest, or they may be independently generated. In military applications, a continuous wave radio frequency source, or jammer, is often aimed at an enemy radar station to purposely disturb the operation of the receiver at that station.
A number of different techniques are used to decrease the interference level in radar receivers. One interference reduction technique, called side-lobe blanking, eliminates false target readings in the receiver. Signals that are transmitted by a particular radar may undergo reflection from objects away from the intended look direction. If the magnitude of such an undesired reflection is great enough, it could be mistaken for a target sighting in the main beam direction. A sidelobe blanker is a device that eliminates such unwanted returns. Other interference rejection techniques are used to eliminate non-pulsed, or continuous-wave (CW), interference. Such interference may be sinusoidal, modulated or noise-like in nature. A jammer or other CW noise source may decrease the dynamic range of the radar receiver. Adaptive nulling is an often used technique for rejecting CW interference. Adaptive nulling rejects such interference by reducing the directional sensitivity of the radar antenna in the interference direction(s).